syscall: attempt to fix windows build, supply missing constant

R=r, brainman, rsc1
CC=golang-dev
https://golang.org/cl/4023043
diff --git a/src/pkg/syscall/ztypes_windows_386.go b/src/pkg/syscall/ztypes_windows_386.go
index e67165f..b1271af 100644
--- a/src/pkg/syscall/ztypes_windows_386.go
+++ b/src/pkg/syscall/ztypes_windows_386.go
@@ -295,9 +295,10 @@
 	AF_INET6   = 23
 	AF_NETBIOS = 17
 
-	SOCK_STREAM = 1
-	SOCK_DGRAM  = 2
-	SOCK_RAW    = 3
+	SOCK_STREAM    = 1
+	SOCK_DGRAM     = 2
+	SOCK_RAW       = 3
+	SOCK_SEQPACKET = 5
 
 	IPPROTO_IP  = 0
 	IPPROTO_TCP = 6